n,m = map(int,input().split())
a = [[0]*(n+1) for _ in range(m)]
for t in range(n*m):
    j,i = map(int,input().split())
    a[i-1][j-1] = max(t-4,0)
    if j==1: a[i-1][n] = max(t-4,0)
ans = n*m
for i in range(m):
    for j in range(n-3):
        b = sorted(a[i][j:j+5])
        t = x = 0
        for k,v in enumerate(b):
            if v<=x: continue
            t += (v-x+4-k)//(5-k)
            x += (v-x+4-k)//(5-k)*(5-k)
        ans = min(ans,t)
print(ans)